It is common in thrust vectoring exhaust nozzles for gas turbine engines to use flaps or other movable surfaces to direct the exhaust gases in order to produce the desired thrust angle and/or exhaust nozzle flow area. In order to achieve reverse thrust, certain thrust vectoring nozzles use a combined valve-flap arrangement wherein the flow diverting flap or flaps is positioned in a blocking arrangement with regard to aftward flow of the exhaust gas while a valving surface or other structure is opened to admit the exhaust gases into an alternate, generally forwardly directed flow passage.
Such reversing systems utilizing an alternate flow path and corresponding valve structure require careful coordination with the blocking flap in order to avoid reducing the total nozzle outlet flow area below a minimum area required to maintain operational stability in the gas turbine engine. If total nozzle outlet area is reduced below such minimum during a reversing maneuver, elevated back pressure at the engine exhaust could induce an engine stall or other undesirable engine instability.
In addition to the timing of the opening of the valve structure, it is also desirable that the rate at which the valve opens meet or exceed the rate at which the blocking flap closes off the aftward nozzle flow area. This is especially difficult to achieve in those nozzle arrangements wherein the valve structure remains normally at rest while the diverter flap moves through a range of orientations for achieving normal forward thrust vectoring and aftward outlet area control.
One further desirable feature for such nozzles is the provision for a safe failure mode wherein the nozzle flaps and valve structure automatically revert to an unvectored, forward thrust orientation upon failure of the nozzle actuators or associated linkages. An aircraft with a failed linkage would thus be operable in a forward thrust mode which in turn enhances the likelihood of recovery from this type of failure.
As with all aircraft components and especially for thrust vectoring nozzles, the weight and complexity of the physical hardware is of prime importance to the designer, with the lightest and simplest arrangement being favored from cost, weight, and reliability standpoints.